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 //===---- TailRecursionElimination.h ----------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file transforms calls of the current function (self recursion) followed
 // by a return instruction with a branch to the entry of the function, creating
 // a loop.  This pass also implements the following extensions to the basic
 // algorithm:
 //
 //  1. Trivial instructions between the call and return do not prevent the
 //     transformation from taking place, though currently the analysis cannot
 //     support moving any really useful instructions (only dead ones).
 //  2. This pass transforms functions that are prevented from being tail
 //     recursive by an associative and commutative expression to use an
 //     accumulator variable, thus compiling the typical naive factorial or
 //     'fib' implementation into efficient code.
 //  3. TRE is performed if the function returns void, if the return
 //     returns the result returned by the call, or if the function returns a
 //     run-time constant on all exits from the function.  It is possible, though
 //     unlikely, that the return returns something else (like constant 0), and
 //     can still be TRE'd.  It can be TRE'd if ALL OTHER return instructions in
 //     the function return the exact same value.
 //  4. If it can prove that callees do not access their caller stack frame,
 //     they are marked as eligible for tail call elimination (by the code
 //     generator).
 //
 // There are several improvements that could be made:
 //
 //  1. If the function has any alloca instructions, these instructions will be
 //     moved out of the entry block of the function, causing them to be
 //     evaluated each time through the tail recursion.  Safely keeping allocas
 //     in the entry block requires analysis to proves that the tail-called
 //     function does not read or write the stack object.
 //  2. Tail recursion is only performed if the call immediately precedes the
 //     return instruction.  It's possible that there could be a jump between
 //     the call and the return.
 //  3. There can be intervening operations between the call and the return that
 //     prevent the TRE from occurring.  For example, there could be GEP's and
 //     stores to memory that will not be read or written by the call.  This
 //     requires some substantial analysis (such as with DSA) to prove safe to
 //     move ahead of the call, but doing so could allow many more TREs to be
 //     performed, for example in TreeAdd/TreeAlloc from the treeadd benchmark.
 //  4. The algorithm we use to detect if callees access their caller stack
 //     frames is very primitive.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
 #define LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H
 
 #include "llvm/IR/PassManager.h"
 
 namespace llvm {
 
 class Function;
 
 struct TailCallElimPass : PassInfoMixin<TailCallElimPass> {
   PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 };
 }
 
 #endif // LLVM_TRANSFORMS_SCALAR_TAILRECURSIONELIMINATION_H

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